Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive (Loss) Income by Component (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Pension and other postretirement benefits (L)
Total Other comprehensive income (loss)	$ (84)	$ 35	$ (472)	$ 122
Foreign currency translation
Other comprehensive (loss) income	(280)	22	(261)	(657)
Cash flow hedges (M)
Net change from periodic revaluations	(222)	(333)	(922)	6
Net amount reclassified to earnings	82	29	201	67
Total Accumulated other comprehensive loss	(5,928)		(5,928)		$ (5,629)
Alcoa Corporation [Member]
Pension and other postretirement benefits (L)
Balance at beginning of period	(2,152)	(2,371)	(2,536)	(2,282)
Unrecognized net actuarial loss and prior service cost/benefit	26	(93)	259	(294)
Tax (expense) benefit	(9)	6	(7)	16
Total Other comprehensive income (loss) before reclassifications, net of tax	17	(87)	252	(278)
Amortization of net actuarial loss and prior service cost/benefit	56	57	206	163
Tax expense	(1)	(2)	(2)	(6)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	55	55	204	157
Total Other comprehensive income (loss)	72	(32)	456	(121)
Balance at end of period	(2,080)	(2,403)	(2,080)	(2,403)
Foreign currency translation
Balance at beginning of period	(2,357)	(2,688)	(2,385)	(2,160)
Other comprehensive (loss) income	(202)	5	(174)	(523)
Balance at end of period	(2,559)	(2,683)	(2,559)	(2,683)
Cash flow hedges (M)
Balance at beginning of period	(1,178)	(221)	(708)	(532)
Net change from periodic revaluations	(222)	(333)	(922)	6
Tax benefit	38	66	169	3
Total Other comprehensive (loss) income before reclassifications, net of tax	(184)	(267)	(753)	9
Net amount reclassified to earnings	82	29	201	67
Tax (expense) benefit	(9)	(2)	(29)	(5)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	73	27	172	62
Total Other comprehensive loss	(111)	(240)	(581)	71
Balance at end of period	(1,289)	(461)	(1,289)	(461)
Total Accumulated other comprehensive loss	(5,928)	(5,547)	(5,928)	(5,547)
Alcoa Corporation [Member] | Aluminum Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	82	21	195	35
Alcoa Corporation [Member] | Financial Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	(3)	3	2	10
Alcoa Corporation [Member] | Interest Rate Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	3	2	7	4
Alcoa Corporation [Member] | Foreign Exchange Contract [Member]
Cash flow hedges (M)
Net amount reclassified to earnings		3	(3)	18
Non-controlling Interest [Member]
Pension and other postretirement benefits (L)
Balance at beginning of period	(63)	(54)	(67)	(56)
Unrecognized net actuarial loss and prior service cost/benefit	14	(7)	16	(7)
Tax (expense) benefit	(4)	2	(4)	2
Total Other comprehensive income (loss) before reclassifications, net of tax	10	(5)	12	(5)
Amortization of net actuarial loss and prior service cost/benefit	2	3	4	5
Tax expense		(1)		(1)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	2	2	4	4
Total Other comprehensive income (loss)	12	(3)	16	(1)
Balance at end of period	(51)	(57)	(51)	(57)
Foreign currency translation
Balance at beginning of period	(853)	(985)	(844)	(834)
Other comprehensive (loss) income	(78)	17	(87)	(134)
Balance at end of period	(931)	(968)	(931)	(968)
Cash flow hedges (M)
Balance at beginning of period	1	(1)	(1)	20
Net change from periodic revaluations	(1)	(1)	(2)	(31)
Tax benefit	1	1	1	9
Total Other comprehensive (loss) income before reclassifications, net of tax			(1)	(22)
Net amount reclassified to earnings	(3)	1	1	2
Tax (expense) benefit	1	(1)		(1)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	(2)		1	1
Total Other comprehensive loss	(2)			(21)
Balance at end of period	(1)	(1)	(1)	(1)
Total Accumulated other comprehensive loss	(983)	(1,026)	(983)	(1,026)
Non-controlling Interest [Member] | Financial Contracts [Member]
Cash flow hedges (M)
Net amount reclassified to earnings	$ (3)	$ 1	$ 1	$ 2